Methods of producing alloys low in carbon



Patented Oct. 28, 1947 93 81 tmrnobs QFiBQD N eet eww 1N 'QARBON 1 iverllennerie t and B0 Mika illl'e-K filtgi himsshp et SW den 'Nq flrawing.gllpplicafion Aprils,-1943, SrialNo.

' 482,890. InSweden March.11,i-1 939 t-Cleims- (c1.17528efiferm-c'chrome e'ailoyss-andhas particular ref: fi

erence :to the treatment .ozfisuch alloys-in which the ferrous :metalcontent :is relatively .high,;a-s for example alloys-obtainable hyreduction oflovv grade;chrcune ores not adaptedto be con-.

verteditb usable ierro-chriome of-low.carhon:con.-. o

tent Joy prior -refiningitreahhents.

, Heretofore'chmmaex es having achrome content 'of the order Jof atleast 60% to 70% have been considered theionly desirable gradesusablefor theupreductiongefiferroechromemfiow:carbon 15 fled e th r wither ..pe -rto1the contentand theproduetidn at this material from areofseventhisliquality:involves expensive refining:processes'rwhichisinceithey are well sknowniin the art needinot :hedeseribed here in. =detail.

Qne :of the required steps,- in such refinement is 29 decarbonization ofthiealley to reduce the carbon content to an acceptabtevaluegparticula'rly when the;alipy istosbeiusedvinlthe'.prodnetionrofstainlesssteel.

.A relatively isiuiizle crsheaap Ilecarburizing 216 process .for mrodneing .L-low :earbbn non-alloyed ferrcns meta'hforflnelting'stock andthelike has heretofore. been-promisedxfiennertelt 11.. S. .Pate entlilp. :;2,1!7!0,1 58) amiinfthefiisclosure of that process; it hasbeen-. smtedzthatthe process .may 80 be.employed;v toflecarhmiizarameng.ether 'materi-.

als, :ferro echromesailoys.

We have.,lhowever,cdiscovered. that.ihe..'e1ecar.-.

burization of ferro-chrome alloys of the :kind now ordinarilyproducedis.::not.,;easib1e..hy :means of 3-5 the process disclosed;-;:*by.-.Rermer,fe1t, which is based essential-1y uponttreatmentin anoxidizing gas atmospheremtjhighltemneizature of the metal to be:deca-rburizedmiththe.metal in subdivided erzpellet :f r=h1;=intorderztemrevide the ratio f 40 s rfac area ateelume-e -l ee e teenable1; gas to act efiectively, he reason for this s hat in such m s he 31chro e. which a ahi h eiygen, s ap d oxidized as wen as the carbon.The-loss of the valuable it.

chrome'al loy is thatfthe o'xidized chrome forms 50 anenyeldpin'g ofairtime oxide covering the surface of the particle under. treatment, andthe nature loft-his oxide is (such that itforms in effect a :film-l-ikeshield which-is impervious to the theusua .ierrwh eme a e issebieeted othe, ahQ e-tre tme tf here imii r ee iq h q- @ueti e en le Id arhe haien t. t e. u fac e pthehar el ea iesi yth imu an u erma ion et hreme e..1? film w iehest i l lfi ntesressi elv-ssiows dew. dziinell eets. to 1stentiail step th his. arm-M e p film acting to substantially stop therea .W h ve hesv r di ee ered het the. deter: bur-w ns ee eh eme alleyunsuccessfu l saseousexidizihs m thqd if the't e tmentses .ere e e etteht daremed w them-a.- terial i edtq the t eatment erwith re pec to th seps eta-the treatm nt, erheth,

We have li l ipdi th the iron c ntent. of a fer wchreme alley"errsuhstantia'lh in reased as c m ar d-withthermaximum content hereto:

fore cons dered practica ly ilsahle, the character of t e ,oxiderfilmiie m d \when the material .is subjected to. an oxidizingeasatmQspherea-p. parently changes .rvery radicall in i nat r Particularly.WithiI'QSlJfiOt to its reaction to; the activitym theoxidizingeas. .Notonly is the content of chromium the oxide reduced incase ofan.:alloy5high;.in;iron:lcontent but werbelieve that, the nature :of theoxide, which is. a complex ironichromeeoxygen compound, possessesessentially different chemical characteristics from those of a chromeoxide of thekin'd formed when a-lfighichrome terroe'chrome alloy issubjectedto oxidizing treatment.

This .adifferenceiin the characteristics of the oxide. surfacefilmappears to, afiectthe gaseous equilibrium zthe 'nxidizihg gasesat.the surface so that the-carbon is the element primarily 0X11? diz'ed.rand-while a certain-amount of chrome is oxidized: the wery 1 much..:gre.ater 1 velocity of diffusion 30f Ethezlcarihonsirnmfthe:jinterior of the particle. toitheesurfaeei than. that.of the chrome enables .Ldecarhurizatienl torbe effectively.accomplishedxwithout. undue Iloss ofs-chrome.

Even more imfiortant than. this, however, is that when the awn contentof thea'lloyis materially increased as. npmapared with that .in thealloys I heretofore: considered practically disable, the nature of thechrome-iron oxide film changes from the iusual?:hamlamdv imperviousnature of a chrome oxide toiagieryious hat-uiie which permits thelozidizingigases to lcontinuecto reactiwith the material-.lbeing.treated.

action ofthesoxidining zas. .a. resultrwhen s5 t v,:wi-th a ieh i-reneontentallrfl decarburization can be carried out with gaseous oxidationto an extent enabling the carbon content of the alloy to be brought downto very low values.

We have found that a ferro-chrome alloy having from 20 to 25% chrome,with the remainder mainly iron and carbon, may readily be decar--burized in a gaseous oxidizing atmosphere to an" 4 or hydrogenatmosphere. Thus we have found it suitable to decarburize a 20% chromealloy having a 5 carbon content down to about 0.25% carbon in anoxidizing atmosphere, thereafter treating the material in a hydrogenatmosphere, by which the carbon content may be brought down to 0.05% orless, while at the same time the oxide film is reduced.

As previously noted the ability to successfully decarburize by reactionwith a gaseous oxidizing atmosphere is apparently due to the perviouscharacter of the oxide formed at the surface of an alloy high in ironcontent and to that end be done by reducing a suitable mixture of chromeU and iron ores with carbon in a furnace, which may be electric or acommon blast furnace. The iron required to produce the desired, highiron content analysis may also be added in metallic form. Obviously, theextent to which iron will be required to be added will depend upon thequality of the chrome ore employed and in the case of ores of extremelylow quality additions of iron (either in the form of ore or as metal)may not be required in order to obtain the high iron content alloysuitable for the decarburizing treatment.

I For the small particle size suitable for the gaseous decarburizingtreatment, the alloy may advantageously be liquified and then granulatedand cooled in water, or it may also be brought to suitable size which isan average particle size of 1.0 to 1.5 millimeters, by crushing.Preferably, particularly'if crushing is employed, the carbon contentofthe initial alloy is relatively high, that is to say, of the order of3% to 5%.

The temperature at which decarburization is carried out should be ashigh as practically possible without risk of sintering and consequentlysticking of the particles to each other or the furnace wall. Thus thetemperature should not materially exceed 1200 C. and a temperature ofthe order of 1100" C. is more to be preferred. Decarburization mayadvantageously be effected by bringing the heated material into contactwith a gas mixture containing 00 and CO2 in suitable proportions,preferably, if the atmosphere contains only these gases, in the ratio ofapproximatelyv 25%-75%. It may be advantageous to oxidize in vacuo or todilute the C0-- C02 mixture with other gases such as H2 or N2, in orderto obtain the most favorable gaseous equilibrium. However, too high aconcentration of nitrogen should be avoided since we have found thatthis gas may become absorbed in the metal. Hydrogen, however, increasesthe velocity of the reaction, and may withadvantage be supplied to thefurnace. In order to supply the quantity of oxygen required todecarburize, without at the same time introducing a supply of nitrogen,it has been found advantageous to introduce steam or water vapor intothe reaction zone, the dissociation of which also acting to enrich theH2 content of the atmosphere. This produces a gas containing H2, H2O,CO, and CO2, andthis gas may be burned with air in the cooler zones ofthe furnace for preheating the charge without risk of absorption ofnitrogen into the alloy.

Because of the fact that an oxide film is formed during the reaction, itmay in any case be advantageous not to carry the decarburization to theultimate degree desired in an oxidizing atmosphere, but to finish theprocess in a reducing the chrome content of the alloy prepared fordecarburizing treatment is advantageously not over about 30% and ispreferably of the order Alloys of higher chrome content are, however,susceptible to gaseous oxidation treatment for decarburizing purposes ifthe nature of the oxide formed. from the higher chrome content alloys istaken into account and the treatment modified accordingly.

Generally speaking, it appears that the higher the chrome contentof the.alloy the more the oxide takes'on the hard tenacious and imperviousqualities of a true chrome oxide. Consequently, if the alloy isrelatively high in chrome. content the reaction, if not entirely stoppedby the oxide film, is retarded to an .extent making the processundesirable if not wholly impractical. -In such instances thedifficultymay, however, be overcome by introducing. what may be termed awashing step into the process. Assuming a relatively high chrome alloyto be treated, the initial gaseous treatment may be the same as for alow chrome contentmaterial. This. will result in a progressive slowing.down of the reaction due to the formation of relatively less perviousfilm, the rate at which the reaction is slowed down depending upon thechrome content of the alloy. When this occurs the character of thegaseous atmosphere is altered by the introduction of a film removing orwashing agent of suitable nature which may for example be chlorine.Removal of the oxide film by the washing agent permits the decarburizingreaction to be resumed, and, by washing. as required, the desired degreeof decarburizationmay be accomplished.

Various forms of furnace may be employed and in many instances it may befound desirable to mechanically agitate the material during thedecarburizing treatment, particularly in the case of treatment of highiron content alloys.

What is claimed;

1. The method of producing low carbon ferrochrome from a high contentchrome ore which consists in preparing in finely divided state an alloyincluding iron and chromium, reducing said highcontent chromeoreincluding the step of adding iron in suitable form to bring the chromecontent of the prepared material within the range of substantially from20% to 30% chrome for subsequent decarburization, and decarburizing saidalloy at a temperature below the melting point thereof.

2. The method of producing lowcarbon ferrochrome which consists inpreparing in finely divided state an alloy including iron and chromiumhaving a carbon content substantially higher than'that desired in theend product, subjecting theresulting material at a temperature below themelting point thereof to decarburization until the decarburizationis'inhibited by an oxide film, removing said oxide film by the use ofchlorine, and thereafter further decarburizing the allow at below themelting point thereof.

3. The method of producing low carbon ferrochrome which consists inpreparing in finely divided state an alloy including iron and chromiumhaving a carbon content substantially higher than that desired in'theend product, subjecting the resulting material at a temperature belowthe melting point thereof to decarburization until the decarburizationis inhibited by an oxide film, removing said oxide film by means of anoxide film removing agent, and thereafter further decarburizing thealloy at below the melting point thereof.

4. The method of producing low carbon ferrochrome from a high contentchrome ore, which consists in preparing in finely divided state an alloyincluding iron and chromium, reducing said high content chrome oreincluding the step of adding iron in suitable form to the ore to reducethe percentage of the chrome content of the prepared material to thedesired low value for subsequent decarburization, and decarburizing saidalloy at a temperature below the melting point thereof to reduce itscarbon content.

5. The method of producing low carbon ferrochrome which consists inutilizing in finely divided REFERENCES CITED The following referencesare of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,170,158 Rennerfelt Aug. 22,1939 96,633 Thoma Nov. 9, 1869 314,004 Edwards Mar. 17, 1885 1,086,019Bucher Feb. 3, 1914 FOREIGN PATENTS Number Country Date 817,471 FranceMay 24, 1937

